Conventionally, in an exposure apparatus used in the manufacture of semiconductor elements and the like, a circuit pattern formed on a mask (reticle) is projection transferred onto a photosensitive substrate such as a wafer via a projection optical system. Resist is coated onto the photosensitive substrate and the resist is sensitized by projection exposure via the projection optical system, obtaining a resist pattern corresponding to the mask pattern.
Thus, the resolution power W of the exposure apparatus is dependent on wavelength λ of the exposure light and numerical aperture NA of the projection optical system and is expressed by the following formula (a).W=K·λ/NA (K: constant)  (a)
Accordingly, in order to improve the resolution power of the exposure apparatus, it is necessary to either shorten the wavelength λ of the exposure light or increase the numerical aperture NA of the projection optical system. In general, from an optics design perspective it is difficult to increase the numerical aperture NA of the projection optical system above a specific value; therefore from this point it is necessary to shorten the wavelength of the exposure light. For example, as an exposure light, an image resolution power of 0.25 μm can be obtained using a KrF excimer laser of wavelength 248 nm, and an image resolution power of 0.1 μm or less can be obtained at a wavelength of 13 nm using, for example, an ArF excimer laser of wavelength 193 nm.
By the way, in the case where X-rays are used as the exposure light, usable transmissive optical materials and refractive optical materials are not available; therefore a reflective-type mask and a reflective-type projection optical system are used. Conventionally, as a projection optical system that can be applied in an exposure apparatus using X-rays as the exposure light, various reflective-type projection optical systems have been proposed, such as Unexamined Patent Application Publication No. H9-211332 Official gazette (application corresponding to U.S. Pat. No. 5,815,310) and Unexamined Patent Application No. 2002-139672 Official gazette (application corresponding to U.S. Pat. No. 6,710,917).
However, in the reflective-type projection optical system disclosed in U.S. Pat. No. 5,815,310 as a conventional example of a reflective-type projection optical system composed of 6 reflective mirrors, all 6 reflective mirrors are configured having aspheric reflective surfaces; therefore, normal aspheric finishing (grinding, polishing) and measurement processes are necessary, causing much effort, time and cost.
In addition, in the reflective-type projection optical system disclosed in U.S. Pat. No. 6,710,917 as a conventional example of a reflective-type projection optical system composed of 8 reflective mirrors, the sixth mirror of the 8 reflective mirrors is formed of spherical surfaces. In an embodiment of the publication concerned, between the second and third reflective mirrors, and then between the sixth and seventh reflective mirrors, an optical system having two intermediate images is used; the sixth reflective mirror is a reflective mirror having a use region (the region where light beams that contribute to imaging are reflected by the reflective mirror, also called “effective region”) in a position farthest from the optical axis. Thus, by forming the sixth reflective mirror, having a use region separated from the optical axis, as a spherical surface, inspection of the sixth reflective mirror is easily done using an interferometer. This is because the farther away the use region is from the optical axis, the harder it is to inspect the degree of asphericity using an interferometer. However, in the reflective-type projection optical system concerned, there is the problem that the effective diameter of the sixth reflective mirror ends up expanding to close to 800 mm. In addition, with respect to as many reflective mirrors as possible, in order to maintain a use region close to the axis, a triple imaging optical system equipped with two intermediate images Z1 and Z2 is used. For that reason, there is also the problem that extension of the distance from the physical object to the image (distance between object and image) to close to 2m cannot be avoided, expanding the size of the entire optical system. In addition, because the reflective mirrors are aspheric surfaces, which demand more stringent manufacturing tolerances, distortion caused by manufacturing tolerances occurs easily when the projection optical system is actually manufactured, and it is difficult to manufacture an optical system that is true to the design.